Justification:
This species is widespread and is important in commercial fisheries throughout its range. Although it is heavily fished, it is considered relatively abundant and is fast-growing, short-lived, and very fecund. It is listed as Least Concern. However, in some regions there may be signs of overfishing and uncertainty in estimating population size and trends. More research and monitoring is needed for this species to develop better population models and to ensure that current fishing mortality does not exceed estimated maximum sustainable yield (MSY).

Indian and Western Pacific OceanThis species is widespread in the Indo-Pacific. It is considered moderately to fully exploited, with no clear indication of decline in catches or estimated biomass, although data in some areas are uncertain. It is listed as Least Concern. However, it is recommended that current fishing mortality does not increase.

Eastern Pacific OceanThis species is wide-ranging in the Eastern Pacific, and there is no evidence to suggest that fishing pressure is negatively impacting the population at present. It is listed as Least Concern.

MediterraneanIn the Mediterranean this species is common but not abundant. Catch statistics indicate a stable fluctuation at low quantities. It is listed as Least Concern.

AtlanticThis species is wide-ranging and a small species, and reproduces at a young age and small size. There is no evidence to suggest that fishing pressure is negatively impacting the population at present. It is listed as Least Concern.

This species is circumglobal in seas warmer than 15°C. In the Eastern Pacific it occurs from British Columbia to northern Chile, including all of the oceanic islands. It occurs in the Mediterranean but is not found in the eastern Mediterranean Sea or the Black Sea. It is found throughout the warm Atlantic, including the Caribbean and the Gulf of Mexico.

Reported worldwide landings fluctuate greatly, but show a gradual increase from 158,600 tonnes in 1950 to 2,480,812 tonnes in 2006 (FAO 2009).

Catch summary statistics for 2000–2004 and the status of the stock (Majikowsk 2007) include the following: 93,000–133,000 tonnes/year in the Eastern Atlantic where the status of the stock is unknown; 22,000–31,000 tonnes/year in the Western Atlantic where the status of the stock is unknown; 422,000–489,000 tonnes/year in the Indian Ocean where the status of the stock is Moderately to Fully exploited; 282,000–439,000 tonnes/year in the Eastern Pacific where the status of the stock is Moderately exploited; 1,136,000–1,295,000 tonnes/year in the Western and Central Pacific where the status of the stock is Moderately exploited.

Eastern Pacific Ocean (EPO)FAO landings data from 1976–2005 vary from approximately 25,000–140,000 tonnes caught annually in the eastern tropical Pacific (FAO 2009). Landings data have been relatively constant over the last 20 years, and between 35,000 and 100,000 tonnes. In general, the Skipjack Tuna global population in the EPO has been high for the last 30 years perhaps related to increased frequency of El-Niño events. Therefore, the general eastward expansion of the Skipjack habitat predicted in this simulation is very plausible (Loukos et al. 2003, Worm and Tittensor 2011).

The last full assessment for Skipjack Tuna in the EPO was in 2005 (Maunder and Harley 2005), although an evaluation of a set of fishery indicators was given in 2008 (Maunder 2010). Reported landings from 1976–2005 (IATTC 2008) varied from 52,000–311,000 metric tonnes per year in the Eastern Pacific including California. The recruitment of tuna to the tropical eastern Pacific region is highly variable and is thought to be the reason for the large variations in stock size. The average weight of this species has been declining since 2000, and the 2007 average weight is approaching the lower reference level. The biomass, recruitment, and exploitation rate has also been increasing over the past 20 years. The main concern with this species' stock is the constantly increasing exploitation rate. However, the data- and model-based indicators have yet to detect any adverse consequence of this increase. There have been observed declines in average weight, and average weight is near its lower reference level, which can be a consequence of overexploitation, but it can also be caused by recent recruitments being stronger than past recruitments (IATTC 2008, Maunder 2010). For example, a group of relatively strong cohorts entered the fishery in 2000–2003.

Western and Central Pacific Ocean (WCPO)Within the equatorial region, fishing mortality increased throughout the modelled period (1952–2007) and is estimated to be highest in the western region in the most recent years. The impact of fishing is predicted to have reduced recent biomass by about 40% in the western equatorial region and 20% in the eastern region (Langley and Hampton 2008). However, this most recent assessment for Skipjack Tuna in the WCPO (Langeley and Hampton 2008) concluded that the stock was not in an overfished state nor was overfishing occurring. Nothing has been observed in the fishery that indicates that this conclusion should be changed, although some mild concern over possible shifts in distribution of Skipjack Tuna in the northern regions of the WCPO has been expressed (Joseph 2009, ISSF 2010).

Indian OceanAn attempt was made in 2003 to assess the stock status but due to the large uncertainties in the information needed to conduct a complete assessment, the Indian Ocean Tuna Commission (IOTC) only analysed different fishery indicators that could provide a general understanding of the state of the stock (i.e., trends in catches and nominal catch per unit effort (CPUE), average weight in the catches, length-based cohort analysis (IOTC 2008, IOTC 2009, STECF 2007). In spite of not having a full stock assessment for this species, the analysis did not show reasons for concern, as catches have continued to increase as effort increased and the majority of the catch comes from fish that are already sexually mature (greater than 40 cm), as the fishing pattern by size indicates. However, length-based cohort analyses indicated a growing catch of smaller size fish due to the fishery with fish aggregating devices (FADs) (STECF 2007).

Preliminary data indicate that catches in 2007 may be the lowest since 2002 (IOTC 2009), possibly due to the lack of fishing in areas patrolled by pirates. However, the range of stock indicators available does not currently signal any problems in the fishery, and catches have continued to increase as effort increased (IOTC 2009, STECF 2009). Furthermore, the majority of the catch comes from fish that are sexually mature (greater than 40 cm) and therefore likely to have already reproduced (STECF 2009). The IOTC has recommended that this species should be carefully monitored given current fishing pressure as it is clear that current catches cannot be increased indefinitely (IOTC 2009).

Atlantic OceanThe Atlantic population is divided into eastern and western stocks; with the eastern stock contributing the majority of catches (~ 80%). Stock assessments for eastern and western Atlantic Skipjack Tuna were conducted in 2008 using available catches to 2006 (STECF 2009). In summary, it appears that both stocks in the Atlantic are currently being fished at levels below maximum sustainable yield (MSY), and the stocks are not being overfished, and are not considered to be in an overfished state (ISSF 2010). The total catches obtained in 2008 in the entire Atlantic Ocean were close to 149,000 t which represents the catch average of the last five years (STECF 2009). This species is not abundant in the Mediterranean with FAO reported landings fluctuating around 100 t from 1996–2005. Although catch is slowly increasing in the Mediterranean in recent years, statistics in the Mediterranean are incomplete (Di Natale pers. comm. 2008).

In the Eastern Atlantic, the preliminary estimates of catches made in 2008 amounted to 127,000 t, representing an increase of 3% as compared to the average of 2003–2007. A Bayesian method, using only catch information estimated the MSY (under a Schaefer-type model parametrisation) at 143,000–156,000 t, a result which agrees with the estimate obtained by the modified Grainger and Garcia approach: 149,000 t. In addition, two non-equilibrium surplus biomass production models were applied for eight time series of CPUEs, and for a combined CPUE index weighted by fishing areas. In general, the range of plausible MSY values estimated from these models (155,000–170,000 t) were larger than in the Bayesian model based on catches. It is therefore difficult to estimate MSY under the continuous increasing conditions of the exploitation plot of this fishery (one-way of the trajectory to substantially weaker effort values) and which as a result, the potential range distribution of some priors needs to be constrained (e.g., for growth rate, or for the shape parameter of the generalized model) (ICCAT 2009, STECF 2009). Although there are discrepancies among various models that create difficulty in estimating MSY, it is unlikely that Skipjack Tuna is over exploited in the eastern Atlantic (ICCAT 2009, STECF 2009).

In the Western Atlantic, the major fishery is the Brazilian baitboat fishery, followed by the Venezuelan purse seine fleet (ICCAT 2009, STECF 2009). Catches in 2008 in the West Atlantic amounted to 22,000 t representing a decrease of 17% as compared to the trend observed for recent years. Catch only model estimated MSY at around 30,000 t (similar to the estimate provided by the Grainger and Garcia approach) and the Bayesian surplus model (Schaefer formulation) at 34,000 t. Other analyses using Multifan-CL indicated MSY convergens to about 31,000–36,000 t. For the western Atlantic stock, in light of the information provided by the trajectories of B/BMSY and F/FMSY, it is unlikely that the current catch is larger than the current replacement yield (ICCAT 2009, STECF 2009).

This pelagic and oceanodromous species is found in offshore waters to depths of 260 m. The larvae are restricted to waters with surface temperatures of 15–30°C in Australia (Kailola et al. 1993). This species exhibits a strong tendency to school in surface waters with birds, drifting objects, sharks, and whales. This species feeds on fish, crustaceans, cephalopods, and molluscs. It is preyed upon by large pelagic fishes (Kailola et al. 1993). In the western Atlantic it is commonly found in mixed schools with Blackfin Tuna, Thunnus atlanticus.

This species is very fast-growing, short-lived, and very fecund. Maximum size recorded is 111 cm fork length (FL) (Bayliff 1988) and 34.5 kg. Longevity is estimated to be between 6–8 years (Garcia-Coll et al. 1986, Collette 2010), and age of first maturity is estimated to be 1.5 years (Maunder and Harley 2005). Size at first maturity is 40–55 cm FL, depending on the area (Collette and Nauen 1983, Matsumoto et al. 1984, Wild and Hampton 1994, Schaefer 2001). Estimated length at 50% maturity for females is 42 cm (Cayre and Farrugio 1986, Stequert and Ramcharrun 1996).

This species sex ratio is about 1:1 but fisheries that rely on young, immature fish are dominated by females, while those that capture older fish are mostly male. This species spawns several times per season in batches: at sea surface temperatures of 24–29°C throughout the year in the Caribbean and other equatorial waters, and from spring to early fall in subtropical waters, with the spawning season becoming shorter as distances from the equator increases (Erdman 1977, Collette 2010). In tropical waters, reproductively active female Skipjack Tuna spawn almost daily. Models of migration have been proposed, especially from the central Pacific into the eastern Pacific. Fecundity increases with size but is highly variable, the number of eggs per season in females 41–87 cm FL ranges from 80,000 to 2,000,000 (Collette 2010).

Skipjack Tuna make up 60% of the commercial tuna catch worldwide and is mostly used for canning. They are taken at the surface mostly with purse seines and pole-and-line gear, but is also caught by trolling on light tackle using plugs, spoons, feathers, or strip bait (Collette 2001).

Eastern Pacific Ocean (EPO)In the EPO, this species is primarily caught with purse seines, and with pole and line and long-line to a lesser degree. It is an important game fish in Panama (D'Croz et al. 1994). More recently this species is being fished with drifting floating objects. The large increase in catch observed in the past 10 years is primarily due to fishing with fish aggregating devices (FADs) in the equatorial eastern Pacific. There is a continuously increasing exploitation rate in the EPO, but models do not predict widespread population decline (IATTC 2008). Given the reduction in larger predators, it is expected that this species is relatively abundant. However, there is a decreasing average weight and increasing catch effort with no comprehensive stock assessment in this region (STEFC 2009).

Western and Central Pacific Ocean (WCPO)Although Skipjack Tuna are the most intensively fished species in the central Pacific Ocean, biomass appears to have remained relatively stable between 1952–1998 (Cox et al. 2002). A Japanese pole-and-line fleet previously dominated the fishery, but it is now dominated by purse seiners, and catches by this gear have shown an increasing trend for three decades (ISSF 2010). Over the past five years, the catch has been at record high levels exceeding 1.2 million tonnes annually and accounting for more than 65% of the annual catch of principal tuna species in the region (STECF 2007). In the WCPO, the level of catch is very high, and some boats are not recording catches, and there are difficulties in monitoring the various fleets concerned (STECF 2009).

Indian OceanIn the Indian Ocean this species is mainly caught by purse seine, gillnet and bait boat (IOTC 2008). The high productivity life history characteristics of Skipjack Tuna suggest this species is resilient and not prone to overfishing, and the stock status indicators suggest there is no need for immediate concern about the status of Skipjack Tuna. However, it is clear that catches cannot grow at the current rate indefinitely (STECF 2007). The effect of FAD fishery on juveniles of other tuna species should be strictly monitored and evaluated.

Atlantic OceanThe numerous changes that have occurred in the Skipjack Tuna fishery since the early 1990s (such as the use of FADs and the expansion of the fishing area towards the west) that have brought about an increase in Skipjack Tuna catchability and in the proportion of the stock that is exploited. At present, the major fisheries are the purse seine fisheries, particularly those of Spain, France, Cape Verde, Guatemala and Ghana, followed by baitboat fisheries of Ghana, Spain and France. Increasing harvests and fishing effort could lead to involuntary consequences for other species that are harvested in combination with Skipjack Tuna. In the West Atlantic, the major fishery is the Brazilian baitboat fishery, followed by the Venezuelan purse seine fleet (STECF 2009). The Brazilian bait boat fleet of Santa Catarina State yields almost half of the Skipjack Tuna catches in the West Atlantic (Andrade and Kinas 2004).With respect to the West Atlantic, the fishing effort of the Brazilian baitboats seems to be stable over the last 20 years. There has been a recent increase in Skipjack Tuna catchability from 1–13% per year since the early 1980s. The change in the selectivity pattern observed for the purse seine fishery suggests that this fleet is mainly targeting juvenile tunas (ICCAT 2008). In the insular Caribbean, this species is important to the artisanal fishery (Mahon 1996). In the Mediterranean, this species may be caught with drift nets.

This is listed as a highly migratory species in Annex I of the 1982 Convention on the Law of the Sea (FAO Fisheries Department 1994).

In the Eastern Pacific Ocean (EPO) previous to 2008, the Inter-America Tropical Tuna Commission (IATTC) had small area-wide closures for 6-weeks on purse-seining for tuna in the eastern Pacific.All purse-seine vessels of more than 182 metric tonnes of carrying capacity that fish in the EPO for Yellowfin, Bigeye, and Skipjack tunas were required to stop fishing in the EPO for a period of 59, 62, and 73 days during 2009, 2010, and 2011, respectively. The closures can be for either of two periods, August–September or November–January (ISSF 2010). Skipjack Tuna is a notoriously difficult species to assess. Due to Skipjack’s high and variable productivity (i.e., annual recruitment is a large proportion of total biomass), it is difficult to detect the effect of fishing on the population with standard fisheries data and stock assessment methods. This is particularly true for the stock of the EPO, due to the lack of age-frequency data and the limited tagging data. One of the major problems mentioned above is the uncertainty as to whether the catch per unit effort (CPUE) of the purse seine fisheries is an appropriate index of abundance for Skipjack Tuna, particularly when the fish are associated with fish-aggregating devices (FADs). Purse seine CPUE data are particularly problematic, because it is difficult to identify the appropriate unit of effort (Maunder 2010). Since the stock assessments and reference points for Skipjack Tuna in the EPO are so uncertain, developing alternative methods to assess and manage the species that are robust to these uncertainties would be beneficial.

In the Atlantic, there is currently no specific regulation in effect for Skipjack Tuna. However, with the aim of protecting juvenile Bigeye Tuna, the French and the Spanish boat owners voluntarily decided to apply a moratorium for fishing under floating objects between November and the end of January for the 1997–1998 and 1998–1999 periods. ICCAT implemented a similar moratorium from 1999 to January 2005. This moratorium has had an effect on Skipjack Tuna catches made with FADs. On the basis of a comparison of average catches between 1993–1996, prior to the moratoria, and those between the 1998–2002 period, the average Skipjack Tuna catches between November and January for the purse seine fleets that applied the moratoria, were reduced by 64%. During that period (1998–2002), the average annual Skipjack Tuna catches by purse seine fleets that applied the moratoria decreased by 41% (42,000 t per year). However, this decrease is possibly a combined result of the decrease in effort and the impact of the moratoria (the average annual catch per boat decreased only 18% between these two periods). This moratorium area was reduced significantly in spatial and temporal coverage in 2001. The ICCAT-SCRS recommends that catches not be allowed to exceed MSY (ISSF 2010, STECF 2009).

In the Western and Central Pacific, there is a limit on the use of FADs and purse seiners, and to reduce the catch of long liners to reduce bycatch for Bigeye Tuna, all of which may have positive conservation effects for this species.

Chur, V.N. and Sharov, V.L. 1984. Determination of age and growth rate of the skipjack tuna, Katsuwonus pelamis (L.) (Scombridae), from the southeastern part of the Gulf of Guinea. J. Ichthyol. 23(3): 53-67.

Collette, B.B. and Nauen, C.E. 1983. FAO Species Catalogue. Vol. 2. Scombrids of the World: an annotated and illustrated catalogue of tunas, mackerels, bonitos and related species known to date. Food and Agriculture Organization of the United Nations (FAO) Fisheries Synopsis number 125, volume 2.

Mahon, R. 1996. Fisheries and research for tunas and tuna-like species in the Western Central Atlantic: Implications of the Agreement for the Implementation of the Provisions of the United Nations Convention on the Law of the Sea of 10 December 1982 Relating to the Conservation and Management of Straddling Fish Stocks and Highly Migratory Fish Stocks. In: FAO (ed.), FAO Fisheries Technical Paper, No. 357. Rome.